Chapter 91: Reading File in Java

Chapter 90 set vocabulary: path, encoding, checked IOException. This chapter is about pulling data in: whole file at once vs streaming line by line when files grow large.

Writing mirrors many APIs—Chapter 92.

1. Topic title

Reading a file: pick charset, pick “all at once” vs “piece by piece,” then close

2. What it means

Small text config: Files.readString(path, StandardCharsets.UTF_8) (Java 11+) returns a String.

Line-oriented text: Files.readAllLines builds a List<String>—simple, but duplicates big files in memory.

Streaming: Files.newBufferedReader(path, charset) or newInputStream for byte pipelines—constant memory if you process while reading.

3. Why it is used

Load settings, parse logs, ingest CSV—any workflow that starts with bytes on disk.

4. Mental sketch

readString is drinking a mug in one gulp. BufferedReader is sipping through a straw—better when the thermos is huge.

5. Java code example

import java.io.BufferedReader;
import java.io.IOException;
import java.nio.charset.StandardCharsets;
import java.nio.file.Files;
import java.nio.file.Path;
 
public class ReadFileDemo {
    public static void main(String[] args) throws IOException {
        Path tmp = Files.createTempFile("read-demo", ".txt");
        Files.writeString(tmp, "line1\nline2\n", StandardCharsets.UTF_8);
 
        String all = Files.readString(tmp, StandardCharsets.UTF_8);
        System.out.println("whole:\n" + all);
 
        try (BufferedReader br = Files.newBufferedReader(tmp, StandardCharsets.UTF_8)) {
            String first = br.readLine();
            System.out.println("first line=" + first);
        }
 
        Files.deleteIfExists(tmp);
    }
}

try-with-resources closes br automatically—Chapter 95 explains the rule; use it now as habit.

6. Explanation of code

readLine returns null at EOF—loop with while ((line = br.readLine()) != null) is the classic pattern.

7. Common mistakes

Default charset on Files.readString(path) overload—explicit UTF_8 avoids “works on my machine” bugs.

Reading gigabytes into one String—switch to streams or channels.

8. Best practices

Centralize charset in a private static final Charset UTF = StandardCharsets.UTF_8 constant if your module reads many files.

Log the path on IOException—support staff need the exact filename.

9. Small practice task

Count non-blank lines in a temp file using BufferedReader only—no readString.

Beginner tip

If NoSuchFileException appears, you looked before the file existed—print path.toAbsolutePath() once; typos hide in relative paths.